1. Field of the Invention
The present invention relates to an optical scanning unit that scans a beam emitted by a light source onto an exposed object, and more particularly, to an optical scanning unit designed to provide sufficient optical path length between a beam deflector and an exposed object to ease installation of an f-θ lens or f-θ lenses and stable optical performance.
2. Description of Related Art
Typically, an optical scanning unit is employed in a laser printer, a digital copier, a facsimile, or the like to produce a latent image on an exposed object through main scanning by a beam deflector and subscanning by rotation of the exposed object.
Referring to FIG. 1, a conventional optical scanning unit includes a light source 1 that produces and emits a beam, a beam deflector 7 that deflects the beam emitted by the light source 1 to be scanned onto an exposed object 15 in the main scanning direction, an f-θ lens 11 that corrects an error contained in the beam deflected by the beam deflector 7. In addition, a collimating lens 3 that collects the divergent beam from the light source 1 and a cylindrical lens 5 that shapes the beam are located in an optical path between the light source 1 and the beam deflector 7, and a mirror member 13 that changes the direction of the scanned beam is disposed between the f-θ lens 11 and the exposed object 15.
The beam deflector 7 includes a driving source 9 and a rotating polygonal mirror 8 rotated by the driving source 9. The direction in which a beam emitted by the light source 1 is scanned is determined according to the reflection direction of a beam that changes as the rotating polygonal mirror 8 rotates.
The f-θ lens 11 includes respective first and second lenses 11a and 11b. First lens 11a compensates for astigmatism present in the beam deflected by the beam deflector 7. Second lens 11b focuses the scanned beam and corrects the beam so that scanning lines are equiangular and equally spaced.
The optical scanning unit configured above has to provide a sufficient optical path between the beam deflector 7 and the exposed object 15 since a beam deflected by the beam deflector 7 is incident onto the entire main scanning area of the exposed object 15. Furthermore, the optical scanning unit requires the f-θ lens 11 that is easy to manufacture and to achieve stable optical performance.
When the optical scanning unit is used in image producing equipment using four exposed objects such as a color laser printer, it is necessary to scan a beam on each object. To simplify the construction of the optical scanning unit while satisfying this requirement, the optical scanning unit must be designed to scan four beams using a single beam deflector. When the optical scanning unit having a single beam deflector scans multiple beams, an optical path length between the beam deflector and the exposed object is too small to allow optical arrangement of four group f-θ lenses.